Recently, synthetic leathers and artificial leathers have come to be substituted for natural leathers in various fields such as shoes, clothes, gloves, bags, game balls and interior decorations. The synthetic leathers and artificial leathers are required to have a high quality, aesthetically pleasing appearance and comfortable feel upon use, and in addition, strongly required to simultaneously have a natural leather-like, delicate appearance, a good flexibility, and particularly in case of shoes, a high peel strength.
Various flexible natural leather-like sheets have been proposed. For example, proposed is a leather-like sheet which is produced by adhesively laminating a resin film having a grain surface resembling natural leathers onto a substrate composed of an entangled nonwoven fabric made of microfine fibers of 0.1 dtex or less and a polyurethane resin coagulated in the nonwoven fabric after impregnation. Such leather-like sheet has a high peel strength enough to use in sport shoes. However, since the surface finished layer is thick and does not fit with the substrate, the natural leather-like properties are not obtained. In addition, it is difficult to form deep grain patterns.
Also proposed is a leather-like sheet which is produced by forming a foamed layer on the substrate as mentioned above by coating a polyurethane solution and wet-coagulating it, then coloring the foamed layer, and finally emboss-processing the foamed layer. Such leather-like sheet has a natural leather-like appearance, but a sufficiently high peel strength is difficult to obtain because a soft elastomer should be impregnated in a relatively small amount when the softness and flexibility are important. If a sufficiently high peel strength is intended, it is necessary to impregnate a relatively hard elastomer in a large amount at the expense of the softness and flexibility. Thus, it has been quite difficult to produce a leather-like sheet which unites all of a good peel strength, flexibility and softness and a natural leather-like appearance.
A leather-like sheet has been proposed, which is produced by impregnating a polyurethane resin into an entangled nonwoven fabric made of microfine fiber-forming fibers, wet-coagulating the polyurethane resin, making the nonwoven fabric into a substrate made of bundles of microfine fibers of 0.1 dtex or less by removing one component from the microfine fiber-forming fibers, and then finishing the surface of the substrate (for example, Document 1). However, the proposed leather-like sheet is insufficient in the flexibility and peel strength, although its surface has a natural leather-like appearance.
Another proposed leather-like sheet has a porous surface layer which is continuously formed on the surface layer of the substrate from the same elastic resin as impregnated into the surface layer (for example, Document 2). However, it is difficult to obtain a sufficient peel strength, if the porous surface layer is formed from a soft resin to obtain a natural leather-like appearance. To obtain a sufficient peel strength, there has been known to form a porous surface layer made of a relatively hard elastomer directly on a substrate, or form a porous surface layer on a substrate via a high-density resin layer, and further, form a porous skin layer on the porous surface layer. When a natural leather-like appearance can be obtained by an emboss processing, such leather-like sheet has a sufficient peel strength, but the softness is deteriorated. If the softness is preferentially provided, a natural leather-like appearance becomes difficult to obtain. When the porous surface layer is formed from a resin which can be easily processed by embossing and the high-density resin layer is formed from a resin which is resistant to deformation by the emboss processing, a satisfactory processability is difficult to obtain in the emboss processing as compared when the porous surface layer and high-density resin layer are formed from the same resin.
To solve the above problem, the inventors have proposed a leather-like sheet having a fibrous base layer closely adhered to a porous surface layer, in which the upper layer of the fibrous base layer is impregnated with a polyurethane resin which is more resistant to thermal deformation as compared with a polyurethane resin forming the porous surface layer (Patent Document 3). To achieve a natural leather-like appearance, it is necessary to emboss-processing a porous surface layer made of a soft polyurethane resin. Therefore, the surface layer and the base layer may fail to be united in some cases when the surface layer is thick.
As mentioned above, a grain-finished artificial leather which combines a good peel strength, a soft surface, a good flexibility, and a natural leather-like, clear and delicate appearance has not yet been provided.
[Patent Document 1] JP 63-5518B
[Patent Document 2] JP 11-140779A
[Patent Document 3] JP 2003-105679A